A typical microstrip or miniature patch antenna has a metallic patch printed on a thin grounded dielectric substrate. In the transmitting mode, a voltage is fed to the patch that excites current on the patch and creates a vertical electric field between the patch and the ground plane. The patch resonates when its length is near .lambda./2, leading to relatively large current and field amplitudes. Such an antenna radiates a relatively broad beam normal to the plane of the substrate. The patch antenna has a very low profile and can be fabricated using photolithographic techniques. It is easily fabricated into linear or planar arrays and readily integrated with microwave integrated circuits.
Disadvantages of early patch antenna configurations included narrow bandwidth, spurious feed radiation, poor polarization purity, limited power capacity and tolerance problems. Much of the development work relating to miniature patch antennas has been directed toward solving these problems.
For example, early miniature patch antennas used direct feeding techniques wherein the feed line runs directly into the patch. Such direct feed arrangements sacrificed bandwidth for antenna efficiency. In particular, while it was desirable to increase substrate thickness to increase bandwidth, this resulted in an increase in spurious feed radiation, increased surface wave power, and potentially increased feed inductance. More recently, noncontacting feed arrangements, such as the aperture coupled antenna have been developed. In the aperture coupled antenna, two parallel substrates are separated by a ground plane. A feed line on the bottom substrate is coupled through a small aperture in the ground plane to a patch on the top substrate. This arrangement allows a thin, high dielectric constant substrate to be used for the feed and a thick, low dielectric constant substrate to be used for the antenna element, allowing independent optimization of both the feed and the radiation functions. Further, the ground plane substantially eliminates spurious radiation from the feed from interfering with the antenna pattern or polarization purity.
Perhaps the most serious drawback of the earlier miniature patch antennas were their narrow bandwidth. Typical approaches to overcome this drawback can be characterized as either using an impedance matching network or parasitic elements.
Notwithstanding the improvements in miniature patch antennas, a need exists for a miniature patch antenna having enhanced radiation efficiency, increased antenna bandwidth and reduced electromagnetic coupling.